Formation of thin-film resistors on silicon substrates

ABSTRACT

The formation of thin-film resistors by the ion implantation of a metallic conductive layer in the surface of a layer of phosphosilicate glass or borophosphosilicate glass which is deposited on a silicon substrate. The metallic conductive layer materials comprise one of the group consisting of tantalum, ruthenium, rhodium, platinum and chromium silicide. The resistor is formed and annealed prior to deposition of metal, e.g. aluminum, on the substrate.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

BACKGROUND OF THE INVENTION

The present invention relates broadly to thin-film resistors and inparticular to the formation of thin-film resistors on silicon substratesby ion implantation.

Direct ion implantation is frequently used in the fabrication process ofintegrated circuits as a useful alternative to high-temperaturediffusion. In this process a beam of impurity ions is accelerated tokinetic energies ranging from several keV to several MeV and is directedonto the surface of the semiconductor. As the impurity atoms enter thecrystal, they give up their energy to the lattice in collisions andfinally come to rest at some average penetration depth. Depending on theimpurity and its implantation energy, the penetration depth in a givensemiconductor may vary from a few hundred angstroms to about 1 μm. Thedistribution of implanted impurities in the semiconductor material isapproximately a gaussian distribution. A uniformly doped region may beachieved by several implantations at different energies.

The obvious advantage of implantation is that it can be done atrelatively low temperatures. The ions can be blocked by metal orphoto-resist layers; therefore, the photo-lithographic techniques may beused to define ion implanted doping patterns. Very shallow (tenths of amicron) and well-defined doping layers can be achieved.

One of the major advantages of ion implantation is the precise controlof doping concentration it provides. Since the ion beam current can bemeasured accurately during implantation, a precise quantity of impuritycan be introduced. This control over doping level, along with theuniformity of the implant over the wafer surface, make ion implantationparticularly attractive for the fabrication of Si integrated circuits.

One problem with this doping method is the lattice damage which resultsfrom collisions between the ions and the lattice atoms. However, most ofthis damage can be removed in Si by heating the crystal after theimplantation. This process is called annealing. Although Si can beheated to temperatures in excess of 1000° C. without difficulty, someother compounds tend to dissociate at high temperatures. By usingannealing methods it is possible to dope Si or compound semiconductorswith good control over doping concentration and with the geometricaltolerances that are required for electronic device fabrication.

The state of the art of thin-film resistors is well represented andalleviated to some degree by the prior art apparatus and approacheswhich are contained in the following U.S. patents:

U.S. Pat. No. 3,833,410 issued to Ang et al on 3 Sept. 1974;

U.S. Pat. No. 4,498,071 issued to Plough, Jr. et al on 5 Feb. 1985;

U.S. Pat. No. 4,520,342 issued to Vugts on 28 May 1985;

U.S. Pat. No. 4,560,583 issued to Moksvold on 24 Dec. 1985; and

U.S Pat. No. 4,597,163 issued to Tsang on 1 July 1986.

Ang et al patent describes thin-film resistors which are made ofthin-film materials that include tantalum. The substrate materialsrecited in this patent are silicon, ceramic, quartz and glass.

Plough, Jr. et al patent is concerned with a high resistance thin-filmresistor with military specifcation stability. It is formed bydepositing a thin metal film on a substrate such as glass.

Vugts patent discusses a thin-film resistor that is formed on chromiumsilicon of an insulating substrate. The resistance range of thismaterial is 100 kilo-ohms to 10 meg-ohms per square.

Moksvold patent discloses a method of forming an integrated resistorelement by ion implantation. The implantation results in a resistor baron a semiconductor wafer.

Tsang patent improves film adhesion between metallic silicide andpolysilicon in thin-film integrated circuit structures by ionimplantation. The patented method includes the steps of depositing ametallic silicide on a substrate and then implanting selected ions atpredetermined doses and energies into the silicide layer whereby tensilestress generated during fabrication processes is reduced.

SUMMARY OF THE INVENTION

This invention pertains to a thin-film resistor structure and the methodof making same for radiation-hardened integrated circuits. The resistoris formed using a thin-film metallic conductor layer such as tantalum orchromium silicide which is deposited by ion implantati,on on the surfaceof fused phosphosilicate glass (PSG) or borophosphosilicate glass (BPSG)substrate. Implantation is at an energy level which provides sufficientpenetration to insure good adhesion and the resistor is annealed attemperatures up to 700° C. The resistor is formed and annealed prior todeposition of metal, e.g. aluminum, on the substrate. Advantages of ionimplantation include very accurate control of dosage, and good adhesionof deposited films.

It is one object of the present invention, therefore, to provide animproved thin-film resistor on a silicon substrate.

It is another object of the invention to provide an improved thin-filmresistor wherein a thin-film metallic conductive layer is deposited byion implantation.

It is still another object of the invention to provide an improvedthin-film resistor wherein ion implantation of a silicon substrate isachieved at an energy level of 20-180 kilovolts.

It is an even further object of the invention to provide an improvedthin-film resistor wherein ion implantation achieves sufficientpenetration of and good adhesion to the substrate material.

It is yet another object of the invention to provide an improvedthin-film resistor wherein the resistor is annealed after implantationat temperatures between 400° to 700° C.

It is still a further object of the invention to provide an improvedthin-film resistor wherein very accurate control of resistor materialdepth and geometry is achieved by ion implantation.

These and other advantages, objects and features of the invention willbecome more apparent after considering the following description takenin conjunction with the illustrative embodiment in the accompanyingdrawing.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE is a top view of a pair of thin-film resistors whichwere formed by ion implantation on a silicon substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Silicon integrated circuits frequently use doped polysilicon as a loadresistor. Typically, the polysilicon layer is obtained by low-pressurechemical vapor deposition, and the doping is obtained by ionimplantation of phosphorus or arsenic.

For radiation-hardened integrated circuit applications, a resistor layerwith a sheet resistivity on the order of 10⁵ ohms per square is desired.Ideally the resistor should have a low temperature coefficient ofresistance, and should not significantly change in resistivity as aresult of total dose radiation such as 10⁶ rads (Si). Both of theserequirements are very difficult to achieve in conventional lightly-dopedpolysilicon resistors.

Referring now to the sole FIGURE, there is shown the typical resistorgeometry for a pair of thin-film resistors 10a, 10b. The thin-filmresistors 10a, 10b are formed on and in the substrate 12 which maycomprise any of the available silicon-based substrates. For the presentexample, the resistor layer for a radiation-hardened integrated circuitmay be formed using a thin-film metallic conductor layer which isdeposited by ion implantation on the surface of a substrate, such asfused phosphosilicate glass (PSG) or borophosphosilicate glass (BPSG).The ion implantation of the resistor geometry occurs prior to the stepsof the aluminum (Al) metal (or alloy) deposition, patterning, alloying,passivation layer deposition, and the bond pad opening. The thin-filmresistor layer would thus be subjected to a temperature on the order of450° C. which is utilized for the aluminum (Al) alloying step. Thesintered aluminum (Al) contacts to the resistor layer would be formedduring this step also.

The resistor layer would be implanted at an energy level which providessufficient penetration to insure good adhesion. This energy level is onthe order of 20-180 kilovolts. The annealing process of the resistor maybe performed at temperatures up to 700° C. without adverse effect. Theresistor geometry may be patterned on the substrate either by etching toremove undesired areas, or by implantation over a patterned resistlayer, which is followed by the removal of the photo-resist prior to theannealing process or step.

The following are a few examples of the possible metallic conductormaterials which may be utilized to form the thin-film resistive layer:tantalum, ruthenium, rhodium, platinum and chromium silicide. However,in the case of tantalum or of chromium silicide resistors, it would benecessary to avoid the process steps, subsequent to the resistordeposition step, which would result in excessive oxidation of theresistor material. The advantages of the use of ion implantation for theformation of thin-film resistors on silicon substrates include veryaccurate control of dosage, and the good adhesion of the depositedfilms.

Since the thin-film resistor formation process puts the resistorgeometries very close to the substrate surface, the pattern which may beused for the contact cuts. does not provide contacts to the ends of theresistors, but only to underlying single-crystal silicon andpolysilicon. After the contact cuts are made, the photo-resist isstripped. A brief etch of the entire wafer surface can be used,immediately prior to metal deposition, to remove oxide which may haveformed over the surface of the implanted thin-film resistor or thesilicon surfaces in the contact cuts. In wet etch processes, a dip in50:1 H₂ O: concentrated hydrofluoric acid for 2 seconds could be used.

The required ion implant dosage depends upon the electrical conductivityof the thin-film resistor after sintering. If it is assumed that theresistor will have, for example, a conductivity of 1 percent of that ofthe pure bulk metal (or compound), then the dosage which is required toachieve a sheet resistance of 100,000 ohms per square, can be estimatedfrom the data on electrical resistivity and the density of bulk metals.On that basis, a dosage of approximately 6×10¹⁴ Ru ions per squarecentimeter would be used for ruthenium resistors.

Although the invention has been described with reference to a particularembodiment, it will be understood to those skilled in the art that theinvention is capable of a variety of alternative embodiments within thespirit and scope of the appended claims.

What is claimed is:
 1. The method of forming thin-film resistors onsilicon substrates comprising the steps of:providing an oxide-coveredsilicon substrate; implanting metallic conductor ions at a predeterminedenergy level into said oxide-covered silicon to form a conductive layer;depositing a photo-resist layer upon said conductive layer; placing apatterned mask upon said photo-resistor layer; exposing saidphoto-resistor layer; removing the unexposed photo-resist layer; etchingto remove undesired areas of said conductive layer; dissolving remainingphoto-resist; applying heat in the temperature range of 450° C. to 700°C. to anneal said conductive layer; and, forming bonding pad on saidconductive layers to form resistive elements.
 2. The method of formingthin-film resistors on silicon substrates comprising the stepsof:providing an oxide-covered silcone substrate; depositing aphoto-resist layer upon said oxide-cover silicon substrate; applying amask and forming a pattern on said photo-resist layer; implantingmetallic conductor ions at a predetermined energy level into saidoxide-covered silicon substrate to form a conductive layer; dissolvingremaining photo-resist layer; depositing an alloy layer on saidoxide-covered silicon substrate and said conductive layer; patterningsaid alloy layer; applying heat in the temperature range of 450° C. to700° C. to anneal said conductive layer; and, forming bonding pad onsaid conductive layers to form resistive elements.
 3. The method offorming thin-film resistors of claim 1 wherein said energy level is inthe range of 20 to 180 kilovolts.
 4. The method of forming thin-filmresistors of claim 1 wherein said conductive layer is one of a groupcomprising: tantalum, ruthenium, rhodium, platinum and chromiumsilicide.
 5. The method of forming thin-film resistors of claim 2wherein said energy level is in the range of 20 to 180 kilovolts.
 6. Themethod of forming thin-film resistors of claim 2 wherein said conductivelayer is one of a group comprising: tantalum, ruthenium, rhodium,platinum and chromium silicide.